1. Field of the Invention
The present invention generally relates to a printed circuit board having at least one pad on which a surface mounted device can be mounted. Particularly, the present invention relates to a pad arrangement on a printed circuit board for applying reworks or engineering changes to an electronic circuits to be formed on said circuit board.
2. Description of the Related Art
The majority of electronic devices are now packaged with their electronic components mounted on printed circuit boards (PCB). PCBs have electrically conductive paths that are often connected to electrically conductive pads upon which surface mounted devices (SMDs) can be mounted. Typically, these SMDs are resistors or capacitors. However, SMDs can also be inductors, jumpers, i.e., a removable wire or small plug whose presence or absence is used to determine some aspect of hardware configuration, or other electrical components known to those of ordinary skill in the art.
Generally, the electrical element of an SMD is carried within a ceramic or plastic package having two conductive end portions that are connected internally to the electrical element and a nonconductive center portion. To permanently connect the SMD electrically to the PCB, its conductive end portions are soldered to appropriately spaced conductive pads provided on the surface of the PCB.
Problems can arise, whenever changes to the layout of the electronic circuit get necessary. May it because of reworks or engineering changes, e.g., due to design requirements or the desire to substitute one SMD for another, the size of an SMD might change. In order to avoid a potentially costly redesign of the PCB layout some arrangements could be made.
U.S. Pat. No. 5,303,122 by Clifford G. Adams, Jr. et. al., assigned to Ford Motor Company, filed May 19, 1993, issued Apr. 12, 1994, xe2x80x9cPrinted circuit board having a commonized mounting pad which different sized surface mounted devices can be mountedxe2x80x9d, discloses a printed circuit board having a unique commonized pad upon which different sized surface mounted devices can be mounted. The preferred form of the commonized pad comprises two electrically conductive pads, each having a tapered portion that electrically connects a narrow portion to a wide portion. The present invention is advantageous over known pads in that it (1) allows for using different sizes of surface mounted devices without redesign and (2) it does not require wasting what might otherwise be antiquated inventory or continuation of an old process to use up what would be antiquated inventory.
In some cases it is more than a question of the size of a SMD. Additional, wires might be required to be connected to the PCB or an additional electrical connection from one surface of the PCB to the other surface may be needed. For this case simple pad arrangements were introduced that would only be used in case of a rework or an engineering change.
Such a pad arrangement 100 is depicted in FIG. 1. As it can be seen from FIG. 1, there is a first conductive pad 102 and a second conductive pad 104. The first conductive pad 102 is formed by a basically rectangularly shaped central portion 106 having truncated corners and a rectangularly shaped extension 108 being provided for receiving a via 110 connecting the first conductive pad 102 to a respective pad (not shown) on the opposite surface of a PCB (not shown). The second conductive pad 104 comprises tree basically rectangularly shaped conductive portions 120, 122 and 124 being connected to each other by two trapezoidal conductive portions 126 and 128, all together forming basically an annulus running approximately half around the first conductive pad 102. The second conductive pad 104 further comprises in one of the trapezoidal conductive portions 128 a via 130 connecting the second conductive pad to a ground line (not shown) provided in the PCB.
Starting from this, the object of the present invention is to provide a pad arrangement on a printed circuit board that allows flexibly applying reworks or engineering changes to an electronic circuits to be formed on said circuit board.
The foregoing object is achieved by a pad arrangement as laid out in the independent claims. Further advantageous embodiments of the present invention are described in the sub claims and are taught in the following description.
According to the present invention a pad arrangement is provided for applying reworks or engineering changes to an electronic circuits to be formed on the circuit board, the pad arrangement comprising a first signal pad for being electrically connected to a first signal line, a second signal pad for being electrically connected to a second signal line, a ground pad being connected to a ground line provided on the circuit board, and a voltage pad being connected to a supply voltage line provided on the circuit board, whereby the pads are arranged in proximity to each other for facilitating a placement of electronic devices between at least two of the pads.
In a preferred embodiment according to the present invention the group of two signal pads, a ground pad and a voltage pad is substantially being arranged in the edges of a square, whereby in a refinement the ground pad and the voltage pad are arranged next to each other.
In order to reduce the inductance on the PCB the connection between the ground pad to the ground line and between the voltage pad to the voltage line are formed basically on the edge of the respective pad which is closest to the respective other of both pads.
According to another embodiment of the present invention the ground pad extends on the respective surface of the circuit board basically around the adjacent signal pad forming further areas for soldering electrical elements onto. In a refinement the ground pad even extends further to the opposite side of the second signal pad, providing an additional shielding from electrical interferences and more possibilities for connecting surface mounted devices (SMD) and electrical wires, such as coaxial cable, to the pads and in particular to the ground pad. A coaxial cable is a kind of cable having a solid central conductor surrounded by an insulator, in turn surrounded by a cylindrical shield woven from fine wires. It is used to carry high frequency signals, in the area of radio frequencies. The shield is usually connected to electrical ground to reduce electrical interference. In the present case the shield may be connected to a portion of the ground pad.
In order to provide further flexibility and to allow a connection from one surface of the PCB to the other, the pad arrangement further comprises in another embodiment a third signal pad being electrically connected to the first signal pad, a fourth signal pad being electrically connected to the second signal pad, another ground pad being connected to a ground line provided on the circuit board, and another voltage pad being connected to a supply voltage line provided on the circuit board, whereby the pads are placed on the opposite surface of the PCB and are again arranged in proximity to each other for facilitating a placement of electronic devices between at least two of the pads. To improve crosstalk behavior, the connection between the first signal pad to the third signal pad and between the second signal pad to the fourth signal pad are formed basically on the edge of the respective pad which is furthest away from the respective other of both pads.
The pad arrangement in accordance with the present invention is placed as prevention of costly and time consuming modifications of cards or PCBs because of potential reworks and engineering changes. It is advantageously applied in order to get connectivity between the front and back side of double side mounted cards and PCBs. In addition, with embedded wires placed in an intermediate layer of the PCB, the pad arrangement according to the present invention may be used, to tunnel blockages or get in to closed areas. Hence, with the design as taught by the present invention one has an tremendous improvement and advantages compared to the known designs.
In addition to the traditional pads, the novel pad arrangement according to the present invention offers particularly the following advantages. The combination of the four pads to a quadrant, with one supply voltage (VCC), one ground (GND) and two signal connections provides the capability and variations to place 0603-SMT (Surface Mounted Technology) components manually or 0805-SMT components automatically on such pads. The variation of placing SMT components within these quadrants can be done in X-direction or Y-direction, to get the functional engineering change (EC) and rework (REA) capability of a serial resistor EC/REA (+2 EC-wire), serial capacitor EC/REA (+2 EC-wire), tie up resistor (+EC-wire), tie down resistor (+EC-wire), capacitor+2 EC-Wires (Highpass), capacitor+1 EC-Wire (Lowpass) or a bypass caps (+up to 2 EC-Wires). The usage of these different EC/REA capabilities can even be combined, except the one with the Serials and the Highpass functions.
The design with all these capabilities is anyhow very small (210 milxc3x97300 mil), so that with one or more of pad arrangements according to the present invention the hazard of card redesigns or scrapping defect cards during manufacturing is much lower. Smaller ground pads adjacent to the signal pads may be used for coaxial cable repair to solder down the respective shield.
The supply voltage and the ground pad via is moved very close to each other and nearby the SMT component contact area, to reduce the overall lead inductance. This will make the pad arrangement according to the present invention also very robust for high frequency design ECs (Engineering Changes) and REAs (Reworks).
Since two signal pads are coming along the novel design (and this very close to each other and very homogenous), one now has the capability to EC/REA also differential pairs without any problems. In short, particularly the following repair scenarios may be possible, Single Net EC/Repair (Front to Rear), Differential Pair EC/Repair (Front to Rear), Resistor EC/Repair, tie-up Resistor+EC-wire, tie-down Resistor+EC-wire, Serial Resistor+2 EC-Wire, Capacitor EC/Repair, Bypass, Capacitor+2 EC-Wires (Highpass), Capacitor+1 EC-Wire (Lowpass) Further a combined usage may be possible within very dense dimensions only 210 mil times 300 mil, furthermore, overlay capability during multiple usage combined usage of the different advantages and test point access capability.